[Client][Server] Add Roots support (#395)

* [Client][Server] Add Roots support

Wire up the MCP "roots" capability on both sides:

Client (php-sdk as client):
- RootsCallbackInterface + ListRootsRequestHandler answer server roots/list
  requests, mirroring the Sampling handler.
- Client::sendRootsListChanged() emits notifications/roots/list_changed.

Server (php-sdk as server):
- ClientGateway::listRoots() requests the client's roots, and
  supportsRoots() reports whether the client advertised the capability,
  normalizing the stored capabilities to an array so an empty
  capabilities object (\stdClass) doesn't cause a TypeError.
- Add ListRootsResult::fromArray() (used by listRoots()).
- ListRootsRequestHandler forwards RootsException messages to the server
  (new Mcp\Exception\RootsException), mirroring SamplingRequestHandler.
- Client::sendRootsListChanged() throws when the client did not advertise
  the roots.listChanged capability.

Examples, docs and tests:
- examples/README/docs advertise
  `new ClientCapabilities(roots: true, rootsListChanged: true)` before
  sending notifications/roots/list_changed.
- CHANGELOG entry for Roots (unreleased 0.7.0 section at the time).
- Unit tests for capability serialization/round-trip, sendRootsListChanged
  gating, and RootsException message forwarding.

phpunit, php-cs-fixer and phpstan green.

* [Client][Server] Address review: schema validation, changelog section, connection guard, working example

Responds to chr-hertel's review of the Roots support:

- ListRootsResult::fromArray() rejects non-array root elements instead of
  accepting malformed entries silently.
- CHANGELOG entry moved to the next unreleased section as the release
  train advanced past it (0.7.0 -> 0.8.0).
- Client::sendRootsListChanged() went straight to
  Protocol::sendNotification(), which sends via
  `$this->transport?->send()` -- on a client that is not connected the
  notification was silently dropped. It now throws a ConnectionException
  like sendRequest() does.
- The stdio example only advertised the roots capability, so the
  roots/list handler never actually ran. The client-communication demo
  server gets an `inspect_workspace_roots` tool that calls
  ClientGateway::supportsRoots()/listRoots(); the example calls it, so
  the server actually issues roots/list and the client answers it.
  Dropped the try/catch wrapper from the example.
- ClientTest's connected-client mock now goes through setState() so
  isConnected() reflects a real handshake instead of a stubbed flag.

phpunit, php-cs-fixer and phpstan green.

src/Client.php

@@ -17,11 +17,13 @@ use Mcp\Client\Protocol;
 use Mcp\Client\Transport\TransportInterface;
 use Mcp\Exception\ConnectionException;
 use Mcp\Exception\RequestException;
+use Mcp\Exception\RuntimeException;
 use Mcp\Schema\Enum\LoggingLevel;
 use Mcp\Schema\Implementation;
 use Mcp\Schema\JsonRpc\Error;
 use Mcp\Schema\JsonRpc\Request;
 use Mcp\Schema\JsonRpc\Response;
+use Mcp\Schema\Notification\RootsListChangedNotification;
 use Mcp\Schema\PromptReference;
 use Mcp\Schema\Request\CallToolRequest;
 use Mcp\Schema\Request\CompletionCompleteRequest;
@@ -244,6 +246,27 @@ class Client
         $this->sendRequest($request);
     }
 
+    /**
+     * Notify the server that the client's list of roots has changed.
+     *
+     * The server should react by requesting an updated list via roots/list.
+     *
+     * @throws RuntimeException    if the client did not advertise the `roots.listChanged` capability
+     * @throws ConnectionException if the client is not connected
+     */
+    public function sendRootsListChanged(): void
+    {
+        if (true !== $this->config->capabilities->rootsListChanged) {
+            throw new RuntimeException('Cannot send a "roots/list_changed" notification without advertising the "roots.listChanged" capability. Build the client with new ClientCapabilities(roots: true, rootsListChanged: true).');
+        }
+
+        if (!$this->isConnected()) {
+            throw new ConnectionException('Client is not connected. Call connect() first.');
+        }
+
+        $this->protocol->sendNotification(new RootsListChangedNotification());
+    }
+
     /**
      * Send a request to the server and wait for response.
      *